Doc | Make authentication methods consistent

What does this MR do?

This makes the details about authentication methods consistent with the parent doc: https://docs.gitlab.com/ee/user/project/repository/mirror/#create-a-repository-mirror.

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doc/user/project/repository/mirror/pull.md

@@ -63,7 +63,7 @@ Prerequisite:
    in the URL, if required: `https://MYUSERNAME@github.com/GROUPNAME/PROJECTNAME.git`
 1. In **Mirror direction**, select **Pull**.
 1. In **Authentication method**, select your authentication method. To learn more, read
-   [Authentication methods for mirrors](#authentication-methods-for-mirrors).
+   [Authentication methods for mirrors](index.md#authentication-methods-for-mirrors).
 1. Select any of the options you need:
    - [**Overwrite diverged branches**](#overwrite-diverged-branches)
    - [**Trigger pipelines for mirror updates**](#trigger-pipelines-for-mirror-updates)